1. FIELD OF THE INVENTION
This invention generally relates to well logging during drilling. The invention more particularly concerns a system and method for determining, while drilling, the concentrations and amounts of one or more different gases in drilling mud returning from the borehole.
2. DESCRIPTION OF THE PRIOR ART
In the drilling of an oil or gas well (the terms "borehole" and "well" being used interchangeably herein), a drill bit is mounted on the end of an elongated rotating drill string which turns the bit and causes it to cut away the underlying earth and rock formations. During this operation, a drilling mud is continuously pumped down through the drill string and into the region around the drill bit and then back up the borehole annulus to the surface. This drilling mud is typically made up of clays, chemical additives and an oil or water base and performs several important functions. The mud cools and lubricates the drill bit, carries drill cuttings back up out of the well, and serves to maintain a hydrostatic pressure which prevents pressurized fluids in the earth formation from blowing out through the drilled well.
During the drilling of a well, various measurements may be taken both of the drilling mud entering the drill string and returning to the surface and of other parameters as determined by tools at or near the drill bit. The measurements at or near the drill bit are typically called measurements while drilling ("MWD") and provide a log of the drilling operations from which one may attempt to analyze the earth formations which the drill bit is penetrating. These logs are important as they enable the drilling operator to ascertain the presence of oil or gas in the formation being drilled. Mud logging measurements, including temperature, electrical conductivity, pH, sulfide ion content and oxidation-reduction potential of the drilling mud returning from the well may also be made. In addition, measurements may be made on the returning mud to ascertain total hydrocarbon content and to ascertain the presence of certain specific gases such as carbon dioxide and hydrogen sulfide in the mud. The gas content of the mud may serve as an indicator of the pore pressure of the drilled section, and if properly determined can be used to identify "oil shows" and "pay zones".
In analyzing the hydrocarbon content of the mud, several techniques have been used. Gas is typically extracted from the mud by mechanical agitation in a gas trap which is located in the possum belly tank (also called "header tank"). The extracted gas is analyzed for "total gas" by one or more of several different detectors such as a catalytic combustion detector (CCD) apparatus, thermal conductivity detectors (TCD), and flame ionization detectors (FID). Separation and quantification of the different light hydrocarbon (i.e. methane through pentanes) gases are then typically carried out via gas chromatography techniques with similar or different detectors. Because chromatography techniques require several minutes for analysis, the gas content of the mud is determined for batch samples taken at discrete intervals of several minutes apart. However, as disclosed in U.S. Pat. #4,635,735 to Crownover, and assigned to the assignee herein, it has been determined that spectrographic analysis of separated gases permits a continuous analysis of the gas content of the mud. In U.S. Pat. #4,635,735, at least a portion of the drilling mud returning from the well is subjected to gas separation in a mud/gas separation means. The separated gas is then subjected to analysis in a gas spectral analyzing means (spectrophotometers) to produce a gaseous component concentration signal whose value at any instant represents the concentration at that instant of the given gaseous component in the separated gas. By also monitoring the flow rate of the returning mud through the separation device, and the flow rate of the separated gas, a continuous determination is made of the concentration of the given gaseous components in the drilling mud. In accord with another aspect of U.S. Pat. #4,635,735, the drilling mud is passed through an agitating type mud/gas separation device while a carrier gas is simultaneously flowed through the mud/gas separation device. The carrier gas is thoroughly mixed in the mud/gas separation device. The resulting mixture of carrier gas and mud gas is separated from the mud in the separation device and is subjected to analysis in a gas analyzer to produce a component gas signal whose value corresponds to the concentration of the component in the gas mixture. By measuring the carrier gas volume flowing into the mud/gas separation device, the flow rate of the mud into the separation device, and the component gas signal, a continuous concentration signal representing the concentration of the component gas in the drilling mud may be obtained.
Despite the advances in the art in obtaining continuous determinations of the concentrations of different gases evolving from the formation, the inventors herein have determined that the final results of all current techniques whether using continuous or batch analyses are flawed due to the sampling and extraction methods utilized in obtaining the analyzed gases. In particular, the inventors have found that the gases obtained by the present techniques may not be representative of the relative gas concentrations evolving from the formation, as large amount of the lighter, more volatile gases (e.g. low carbon numbers) which are more typically found as gas bubbles in the mud may be lost at the bell nipple. In addition, the gases which are finally extracted from those that remain in the mud are also extracted as a function of their solubility and volatility (carbon number) due at least partly to the fractionation processes between individual hydrocarbons that takes place during transport to the surface. As a result, in many situations only a small proportion of the gas evolving from the formation may actually be measured by the known measurement techniques, and the measured portion is not representative of the gas composition of the formation.